Magneto ignition systems are based upon the electrical principle that voltage is generated in any conductor which is subjected to a change in magneto flux through the conductor. More specifically, a sudden collapsing of the magnetic flux in the core upon which a conductor is mounted will induce a high voltage which can be applied to a spark gap for fuel ignition.
The conventional ignition systems for internal combustion engines have used cam actuated breaker points. The breaker points physically break the magneto coil circuit to induce a high voltage at the proper time in the engine cycle to cause sparking action at the spark plug. With the advent of solid-state switching circuits, many designers in the ignition art recognized the advantages of substituting such circuits for the breaker points. Various electronic circuits, including transistors and silicon controlled rectifiers (SCR), were used in place of the breaker points to interrupt the current to the magneto or primary winding. The use of an auxiliary pick off coil to trigger the switching action of the electronic circuit also was implemented as an appropriate means to control the timing of the switching action.
The first step in the evolution of breakerless magneto ignition systems was to connect a semiconductor device to the primary winding of the magneto coil and rely upon the coil to carry out its basic function of supplying a high voltage to the spark plug and also to provide the additional function which had formerly been performed by the breaker points.
When the magneto coil was used to provide both functions, a design tradeoff was necessary. If the solid-state device was performing its functions perfectly, i.e., allowing no voltage to develop across the primary terminals, then there would be no voltage signal present from which a spark timing signal could be derived. If a voltage were allowed to develop across the primary (which in the practical solid-state situation always occurs) then the efficiency and effectiveness of the spark system was drastically reduced. In either case, a problem would exist in that the ideal instant to interrupt the primary circuit, thereby collapsing the flux field and inducing a high voltage in the secondary circuit, is at the moment of maximum primary current. Any scheme combining the circuit interrupting function and the timing function on a single coil winding thereby necessitated a design compromise.
One solution to the problem was the introduction of a separate trigger winding mounted with the magneto coil on a single magnetic core. With a separate winding on the same core there was no longer unity coupling between the primary winding and the trigger winding. With this construction a portion of the voltage induced in the added winding was generated by magnetic flux that did not contribute to the current flow in the short circuited primary. It is clear that if the coupling between the two windings, the primary winding and the trigger winding, were complete, the additional winding would produce no different results than what had previously been obtained by using the primary winding for both functions.
But even with a separate trigger winding mounted on the same core, the performance of these magneto ignition systems was not satisfactory. Often the resulting spark was erratic and unstable in both amplitude and time.
The present invention overcomes these defects and produces a better, more stable ignition system. The primary and trigger windings are mounted on separate, cores. The core upon which the auxiliary trigger coil is mounted is necessarily located close to and adjacent the main magnetic core of the magneto for reasons of spark timing and is operationally substantially isolated magnetically. By adjusting the spacing between the auxiliary trigger core and the main core, the instant of spark occurrence with respect to maximum primary current can be independently controlled. Spark timing with respect to piston position (or crank angle) is controlled by angular orientation of the entire magneto assembly.